In recent years, accompanying the adoption of multimedia information in cellular mobile communication systems, it has become common to transmit not only speech data but also a large amount of data such as still image data and moving image data. Furthermore, studies are being actively conducted in LTE-Advanced (Long Term Evolution Advanced) to realize high transmission rates by utilizing broad radio bands, Multiple-Input Multiple-Output (MIMO) transmission technology, and interference control technology.
In addition, taking into consideration the introduction of various devices as radio communication terminals in M2M (machine to machine) communication and the like as well as an increase in the number of multiplexing target terminals due to MIMO transmission technology, there is a concern regarding a shortage of resources in a region (that is, a “PDCCH region”) to which a PDCCH (Physical Downlink Control Channel) to be used for a control signal is assigned. A DL grant (also referred to as “DL assignment”), which indicates a downlink (DL) data assignment, and a UL grant, which indicates an uplink (UL) data assignment, are transmitted on a PDCCH. The DL grant notifies the terminal that a resource in the subframe in which the DL grant is transmitted has been assigned to the terminal. The UL grant notifies that a resource in a target subframe defined in advance by the UL grant has been assigned to the terminal. If a control signal (PDCCH) cannot be assigned due to such resource shortage of the PDCCH region, downlink data cannot be assigned to the terminals. Therefore, even if a resource region (i.e., a “PDSCH (Physical Downlink Shared Channel) region”) to which downlink data is to be assigned is available, the resource region may not be used, which causes a decrease in the system throughput.
As a method for solving such resource shortage of the PDCCH region, a study has been carried out on arranging, in a data region, control signals for radio terminal apparatuses (hereinafter, abbreviated as “terminal” and also referred to as “UE (User Equipment)”) served by a radio base station apparatus (hereunder, abbreviated as “base station”). A resource region in which control signals for terminals served by the base station are assigned is referred to as an Enhanced PDCCH (ePDCCH) region, a New-PDCCH (N-PDCCH) region, an X-PDCCH region or the like. Assigning the control signal (i.e., ePDCCH) in a data region as described above enables transmission power control on control signals transmitted to a terminal near a cell edge or interference control for interference by a control signal to another cell or interference from another cell to the cell provided by the base station.
A study has been carried out on assigning an ePDCCH to a logical resource, which is referred to as eCCE (enhanced Control Channel Elements), and then assigning the ePDCCH to a physical resource (for example, see FIG. 1). In the LTE and LTE-Advanced systems, one RB (resource block) has 12 subcarriers in the frequency domain and has a width of 0.5 msec in the time region (for example, see NPL 1). A unit in which two RBs are combined in the time region is referred to as an RB pair (for example, see FIG. 1). That is, an RB pair has 12 subcarriers in the frequency domain, and has a width of 1 msec in the time region. When an RB pair represents a group of 12 subcarriers on the frequency axis, the RB pair may be referred to as simply “RB.” In addition, in a physical layer, an RB pair is also referred to as a PRB pair (physical RB pair). A resource element (RE) is a unit defined by a single subcarrier and a single OFDM symbol (see FIG. 1).
The number of eCCEs that are used to transmit the ePDCCH is referred to as an aggregation level. The base station determines the application level according to the channel quality between the base station and the terminal.
A set of assignment candidates (ePDCCH candidates) of a resource region, to which the ePDCCH is assigned, is referred to as a search space. The search space for the ePDCCH is configured for an individual terminal by higher layer signaling. As a method for higher layer signaling, a study has been carried out on designating the number of a PRB pair corresponding to the search space among the PRB pairs as a physical resource unit. The terminal recognizes, as the search space of the terminal, PRB pairs that are identified by the PRB pair number notified by higher layer signaling, a configuration pattern (the aggregation level, the number of ePDCCH candidates of each aggregation level, a shift pattern, and the like) separately defined in advance. The terminal monitors the search space of the terminal to detect an ePDCCH intended for the terminal.
“Localized assignment” that assigns ePDCCHs collectively at positions close to each other on the frequency band, and “distributed assignment” that assigns ePDCCHs by distributing the ePDCCHs on the frequency band are being studied as assignment methods for ePDCCHs (for example, see FIG. 1). Localized assignment is an assignment method for obtaining a frequency scheduling gain, and can be used to assign ePDCCHs to resources that have favorable channel quality based on channel quality information. Distributed assignment distributes ePDCCHs on the frequency axis, and can obtain a frequency diversity gain. In the LTE-Advanced system, both a search space for localized assignment and a search space for distributed assignment may be configured.
In the localized assignment, each eCCE may be assigned in a unit in which a PRB pair is divided into four, three, or two eCCEs. In the localized assignment, when the aggregation level is equal to or larger than 2, a plurality of eCCEs to which an ePDCCH is assigned are assigned to the same PRB pair. However, when the aggregation level is larger than the number of divisions of the PRB pair, the eCCEs are assigned to a plurality of PRB pairs.
In the distributed assignment, the eCCEs are assigned to a plurality of PRB pairs. A resource (RE group) that is obtained by dividing a PRB pair is referred to as an eREG (enhanced Resource Element Group), and one eCCE is assigned to a plurality of eREGs that belong to different PRB pairs. As a method of dividing a PRB pair into eREGs, there is a method of dividing a PRB pair in subcarrier units, a method of generating and dividing a resource (RE) group, or the like. The number of divisions of a PRB pair (the number of eREGs per PRB pair) may be 8, 12, 16, 24, 36, and the like (for example, see FIGS. 2A to 2C; when the number of divisions is 8, 16, and 36).
The base station can configure the search spaces of a plurality of terminals in the same PRB pair. Since a minimum unit for transmitting an ePDCCH becomes a resource region smaller than a PRB pair, the ePDCCHs of a plurality of terminals are arranged in the same resource region or in different resource regions within the same PRB pair, thereby reducing the number of PRB pairs for the ePDCCH and increasing the number of PRB pairs for data. Accordingly, there is a need for a method of configuring a search space that can be shared by a plurality of terminals.
In the LTE-Advanced, a study has been carried out on configuring a plurality of search spaces of ePDCCHs for each terminal. For example, as a case in which a plurality of search spaces of ePDCCHs are configured for each terminal, the following cases (1) to (3) can be identified.
(1) Common Search Space and UE Specific Search Space
A common search space that is used to transmit special control signals and a UE specific search space into which a DL assignment and a UL grant of an individual terminal are transmitted are configured in each terminal. The special control signals include system information, paging, RACH response, PUDCCH power control, PUDSCH power control, and the like, and control signals masked with SI-RNTI, P-RNTI, RA-RNTI, TPC-PUCCH-RNTI, and TPC-PUSCH-RNTI are transmitted.
(2) Two UE Specific Search Space
Two UE specific search spaces (for example, search space 1 and search space 2) of each individual terminal are configured (for example, see NPL 3). For example, the search space 1 is used for more robust transmission than the search space 2. The robust transmission is attained by using a PRB pair in which interference control with other cells is performed, configuring the position of an ePDCCH candidate having a high aggregation level, configuring the position of an ePDCCH candidate having a high frequency, space, or time diversity order, or the like.
An operation becomes possible, where, by assigning the search space 1 of the two search spaces so as to be shared between a plurality of terminals, when the number of terminals per subframe is small, only the search space 1 is used, and when the number of terminals increases, the search space 2 is further used. By adapting this configuration, when the number of terminals is small, it is possible to reduce the number of PRB pairs for an ePDCCH, thereby increasing the number of PRB pairs that can be used for data transmission. At this time, the search space 2 varies for each terminal and a terminal in which the search space 2 is used is selected to change the PRB pairs for the ePDCCH, thereby improving flexibility of the use of the PRB pairs.
The search space 1 may be for distributed assignment, and the search space 2 may be for localized assignment. In this case, when the reliability of feedback information is high, the search space 2 (localized assignment) is used, and PRB pairs having excellent characteristics inherent in the terminal can be assigned. When reliability of feedback information is low, switching to the use of the search space 1 (distributed assignment) is made to obtain the frequency diversity gain, and the PRB pairs are shared with other terminals, thereby improving the utilization efficiency of the PRB pairs.
(3) Band Extension Function (CA: Carrier Aggregation) and Cross Carrier Scheduling
The CA is a function that is newly introduced in the LTE-Advanced, and brings a plurality of LTE system regions called component carriers (CC) together, thereby realizing improvement of the maximum transmission rate (see NPL 2). When a terminal uses a plurality of CCs, one CC is configured as a primary cell (PCell), and the remaining CCs are configured as secondary CCs (SCell). The configuration of the PCell and the SCells may vary for each terminal. The cross carrier scheduling is a resource assignment method in which inter-cell interference control is performed in a CC unit in a PDCCH. In the cross carrier scheduling, the base station can transmit DL grants and UL grants of other CCs in the PDCCH region of a certain CC. If the cross carrier scheduling is applied, the PDCCH is transmitted from different CCs between adjacent cells, thereby reducing inter-cell interference of the PDCCH.
During the CA operation, when the cross carrier scheduling is configured, controls signals for a plurality of CCs are collected into one CC, and a plurality of search spaces corresponding to each of a plurality of CCs (PCell and SCells) are configured.
The cases (1) to (3) where a plurality of search spaces of ePDCCHs for each terminal have been described.
In the LTE, as a feedback method (A/N mapping method) of a response signal (ACK/NACK signal, A/N signal) for downlink data assignment, A/N mapping (PUCCH (Physical Uplink Control CHannel) format 1a: BPSK) during one code word (CW) processing and A/N mapping (PUCCH format 1b: QPSK) during 2CW processing are adopted.
When a DL assignment is transmitted using a PDCCH, a resource (PUCCH resource) of the PUCCH is defined in association with a CCE index of a first CCE among CCEs (Control Channel Element) as a resource used to transmit the DL assignment on a one-to-one basis. The PUCCH resource is called an implicit resource. The CCEs are resources that are generated by dividing the resource (PDCCH resource) of the PDCCH, and are attached with CCE indexes that do not overlap each other. A CCE index is recognized commonly between terminals within a cell.
The DL assignment and the UL grant are assigned to one CCE (Aggregation level: 1) or a plurality of CCEs (Aggregation level: 2, 4, 8) according to the aggregation level to be set. When control information is assigned to a plurality of CCEs, the control information is assigned to CCEs having continuous CCE indexes.
In the LTE, the PDCCH is demodulated with a CRS (Cell-specific Reference Signal) as a reference signal. An antenna port of a CRS to be used is common between terminals within a cell. Accordingly, it is difficult to apply MU-MIMO (Multi user MIMO) in which a plurality of DL assignments or UL grants are transmitted by the same CCE.
Since one PDCCH region is configured per cell, there is no case where the CCE indexes of the CCEs to which the DL assignment and the UL grant are assigned overlap between terminals within a cell. That is, when a terminal within the cell uses the PDCCH region, it is designed such that there is no collision of the PUCCH resources associated with the CCE indexes.
However, exceptionally, when the PUCCH is transmitted through two antenna ports and when channel selection is applied to the PUCCH, two PUCCH resources are implicitly designated from the CCE indexes that are used for one DL assignment. For example, PUCCH resources respectively associated with first CCE index #N and next CCE index #N+1 are used. Here, if the DL assignment of the other terminal is assigned using the CCE of CCE index #N+1, there is collision of the PUCCH resources that are used between terminals. The base station does not use CCE index #N+1 for a DL assignment or a UL grant is assigned to CCE index #N+1, thereby avoiding collision of the PUCCH resources. Since there are many cases where a DL assignment that indicates transmission of two CWs has an aggregation level equal to or larger than 2, collision of the PUCCH resources does not become a significant problem as much. In this way, in the PDCCH, since there is no case where the CCE indexes to be used collide within the same cell, collision of the PUCCH resources does not become a major problem.